The present study aims to show how a diachronic approach may be used to characterize what may be considered as a heterogeneous group, i.e. American think tanks. It is based on the following hypothesis: it is only in the 1970s that think tanks started crystallizing into a specialized milieu, under the influence of various evolutions that have marked America’s recent history. The milieu is then defined by the same objective, practices and discursive genres. After a brief review of potential methodological approaches that could help trace the conditions of their emergence in the course of the 20th century, a historical and cultural perspective is offered. It focuses on the role individuals and underlying historical currents may have played in the transition from heterogeneous research organizations to a distinctive social category. Two crystallizing factors are identified, one of which is the evolving role of experts in the US political debate. Paradoxically, these historical factors account for the hybridity of the think tank milieu which may thus be seen as key to its definition process.